Disregard if this isn’t a bug but it certianly seems bug-like.
I’ve noticed that some mobs (usually those that use Leap or Bite) will use two attacks instantly.
Another one that comes to mind is a Champion (can’t remember the name) that does a large AoE ranged attack. Sometimes when hit by the attack you will take damage twice resulting in instant death (e.g. as a level 80 downlevelled, I was twice for 6k each and only had about 7k hp).